handle nil configuration returned by workspace/configuration #1051

Job	Run time
static analysis (Elixir 1.15.x \| Erlang/OTP 25.x)	18s
mix test windows (Elixir 1.13.x \| Erlang/OTP 22.x)	7m 23s
mix test (Elixir 1.13.x \| Erlang/OTP 22.x)	8m 16s
mix test windows (Elixir 1.13.x \| Erlang/OTP 23.x)	7m 25s
mix test (Elixir 1.13.x \| Erlang/OTP 23.x)	9m 2s
mix test windows (Elixir 1.13.x \| Erlang/OTP 24.x)	8m 34s
mix test (Elixir 1.13.x \| Erlang/OTP 24.x)	7m 28s
mix test windows (Elixir 1.13.x \| Erlang/OTP 25.x)	8m 41s
mix test (Elixir 1.13.x \| Erlang/OTP 25.x)	5m 10s
mix test windows (Elixir 1.14.x \| Erlang/OTP 23.x)	8m 29s
mix test (Elixir 1.14.x \| Erlang/OTP 23.x)	10m 45s
mix test windows (Elixir 1.14.x \| Erlang/OTP 24.x)	8m 47s
mix test (Elixir 1.14.x \| Erlang/OTP 24.x)	8m 20s
mix test windows (Elixir 1.14.x \| Erlang/OTP 25.x)	7m 16s
mix test (Elixir 1.14.x \| Erlang/OTP 25.x)	5m 9s
mix test windows (Elixir 1.15.x \| Erlang/OTP 24.x)	6m 50s
mix test (Elixir 1.15.x \| Erlang/OTP 24.x)	6m 9s
mix test windows (Elixir 1.15.x \| Erlang/OTP 25.x)	5m 54s
mix test (Elixir 1.15.x \| Erlang/OTP 25.x)	4m 11s
	2h 14m 7s

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